In order to solve the electromagnetic design problems of Permanent Magnet(PM) brushless DC motors in reaction wheels

an objective design method based on mechanical characteristics of motors was presented by using Finite Element Method (FEM) combining with a magnetic circuit method. Firstly

according to the structural characteristics

large equivalent air-gaps

and the large ratio of diameter to length of a motor

the corresponding relationship among mechanical characteristics

equivalent electromagnetic air-gap and armature diameters was derived

and a new formula of calculating the armature diameters was obtained. Then a model for calculating magnetic steel sizes was given by the strategy of magnetic field inverse problem and the method of equivalent air-gap magnetic flux density. Finally

the whole electromagnetic design process was described. Compared with a PM brushless DC motor with a angular moment of 5 Nms and the highest speed of 6 000 r/min in a reaction wheel

the maximum error by proposed method is 2.89%. Experimental results indicate that the electromagnetic design method based on objective design has higher design precision and speed and it is suitable for the electromagnetic design of motors with larger magnetic air gaps and larger ratios of diameters to lengths in reaction wheels.